Simoleon

Nobody really knows how this term came into being, but it's probably a portmanteau word made up by combining a 'simon' and a 'napoleon'.

A napoleon, as you probably know, was the colloquial name for a French gold coin, such as those that were originally minted for the Emperor Napoleon I and bore his portrait. A simon was British slang for a sixpenny coin – possibly after Thomas Simon, a famous seventeenth–century engraver at the Royal Mint, who designed some new coins after the Restoration in 1660, including the sixpence.

Incidentally: in the Bible (Acts 10:5–6), a man called Cornelius is told by an angel to "call for one Simon, whose surname is Peter", who "lodgeth with one Simon a tanner". And that's how a sixpenny coin (previously known as a simon) came to be known as a tanner.

Anyway ... as for the simoleon, there are a couple of American references dating to the 1880s. The estimable World Wide Words surmises that "some wit, familiar with the word's slang sense and with the valuable French gold coins called Napoleons, bundled simon and Napoleon together and made from it simoleon."